ACTINOPTERYGII 



33 



\rr 



Fio. 278. 



Ventral view of the pelvic girdle and fins of 

 kel 



The pectoral fin has a fairly normal skeleton, conforming to the 

 rhipidostychous plan, with 

 a postaxial axis (Fig. 279) ; 

 but the pelvic fin has a 

 remarkable structure, differ- 

 ing from that of all other ^ 

 Osteichthyes. No definite ' 

 line of demarcation exists 

 between the pelvic girdle 

 and the skeleton of the fin 

 itself (Figs. 276-8). The 

 radials, forming a single 

 series, are articulated to 

 basals, which are more or 

 less independent posteriorly, 

 but become fused to the 

 girdle in front. A skeleton 

 is thus formed somewhat 

 resembling that of Clado- 

 selache, and suggesting a 

 derivation from a primitive 

 orthostichous type (Wieders- Acipenser Sturi0) L . ^ 8 - keleton - of the right fin 



heim [4921, Regan [3451) completely exposed, dr, lepidotrichia supporting 

 , , L , , J . L ~l web of fin ; "p, pelvic cartilage ; pr.r, preaxial radials. 



rather than from an out- (From Quart. Joum. Micr. sti.) 



standing axis in a projecting 



lobe (see p. 108). Dorsal processes, analogous if not homologous 



with the iliac process, are 

 developed along the basal 

 region of the fin of Polyo- 

 don. In Scaphirhynchus 

 cataphractus the internal 

 end of the pelvic cartilage 

 may be segmented off as a 

 separate element. So far 

 as is known, the radials 

 of the dorsal and anal fins 

 undergo little concentra- 

 tion, and do not tend to 

 fuse together proximally 

 FlQ - 279. as i n the lower Teleostomi 



Pectoral girdle and fins of Acipenser sturio. Ventral and the Dipnoi. The tail 

 , ..-__ Tr..._, ,.__.. ^ n postaxial . , . , , , 



cieithrum ; F, is heterocercal, nomocercal, 

 preaxial dermal or gephyrocercal, never 

 truly diphycercal (Belono- 

 rhynchidae ?). When the median fin is subdivided, as is the 

 case in all except some Teleostei, a single anal and a single 



ci 



Cl' 



ray ; r, 



